1. Field of the Invention
This invention relates to high performance analog multiplexers. 2. Brief Description of the Prior Art
When large quantities of analog data are generated on chip, such as in the case of a focal plane array (FPA), it is necessary to time-domain multiplex (MUX) the signals so that the data can be read serially off chip. With the recent and continuing improvements in detector performance (use of photodiodes such as vertically integrated photodiodes (VIP) instead of photocapacitors, such as vertically integrated metal insulator semiconductors (VIMIS)), the performance of the CMOS circuits that process the detector output also require considerable improvement in order to make full use of the improved detectors.
One standard analog multiplex solution has been the use of a common load source follower multiplexer. A two transistor source follower amplifier is used with a multiplexing switch in each branch of the circuit. The current source transistor of the amplifier is common to all of the input transistors in each of the branches and therefore constantly supplies power to only the branch in use, which minimizes the power consumption. This configuration has two serious second order problems and at least a half dozen performance deficiencies, among these being scan direction crosstalk and positive feedback which causes the amplifier to run significantly slower than predicted.
Another solution is to use one operational amplifier per branch (5 transistor differential amplifier in unity gain mode, for example). This delivers the performance necessary but uses excessive power since each multiplex stage is using full power at all times. Minimizing power consumption is always desirable, but when working at cryogenic temperature, it is absolutely necessary.
It is therefore apparent that an improved analog multiplexer which eliminates one or more of the deficiencies of the prior art analog multiplexer is highly desirable.